A rich structural chemistry in π-conjugated hydroisocyanurates: layered structures of A2B(H2C3N3O3)4·nH2O (A = K, Rb, Cs; B = Mg, Ca; n = 4, 10) with high ultraviolet transparency and strong optical anisotropy

Abstract

Ultraviolet (UV) transparent birefringent crystals are indeed indispensable for modern optoelectronics and polarizer devices. Herein, mixed alkali/alkali-earth metal hydroisocyanurates A₂B(H₂C₃N₃O₃)₄·nH₂O (A = K, Rb, Cs; B = Mg, Ca; n = 4, 10) were synthesized and their thermal, vibrational and optical properties were characterized in detail. Although they crystallize in different crystallographic point groups, all compounds feature quasi-two-dimensional layered structures built by the ²_∞[H₂C₃N₃O₃]⁻ ribbons through hydrogen bonds, separated by water molecules and cations. Benefiting from the delocalized π-conjugated bonds and planar structural configuration of (H₂C₃N₃O₃)⁻ groups, they show a concurrently short ultraviolet absorption edge (λ_cut-off ≈ 230 nm) and strong optical anisotropy (Δn ≈ 0.37 at 800 nm), twice larger than that of the benchmark UV birefringent crystal calcite.